1. Field of the Invention
The present invention relates to the manufacture of linear position encoders, and more particularly to a low-cost method of manufacture which avoids the use of collimating optics.
2. Description Relative to the Prior Art
Position encoders are used in a variety of position measuring equipment. Such encoders contain a large number of closely-spaced rulings parallel to each other.
An error-free linear encoder is an encoder in which the pitch, or spacing between rulings, in constant from end to end. In the past, such encoders were either ruled in the same manner that diffraction gratings were ruled, or produced lithographically by reducing a large-scale drawing of the encoder and using a step-and-repeat process to make a long encoder. U.S. Pat. No. 4,219,933 is an example of the ruling approach. The use of lithography in forming diffraction gratings is described by Akedo et al., Formation of a Difftactional Grating by Magnetic Lithograph, Nippon Oyo Jiki Gakkaishi, vol. 13, No. 2, 1989.
More recently, holography has offered an attractive alternative as a technique for making uniform encoders. In this technique, light from two coherent point sources is collimated and the resulting beams are made to overlap in the region where the holographic encoder is to the exposed. FIG. 1 depicts an apparatus used in the holographic approach. Referring to FIG. 1, two coherent light sources 2, 4 radiate light cones 6,8 which are reflected off collimating mirrors, which create an overlapping region 14 in which interference fringes are produced. A hologram plate 16 is disposed within the area of the fringes.
When the interference fringe pattern is generated, a series of bright bands appears where the two beams constructively interfere, and alternate dark bands where they destructively interfere. The pattern is generated in this way in the plane of the hologram plate, which may be a photographic plate, or other photosensitive surface. In the case of a photographic plate, the exposed emulsion is then fixed and hardened in an acid bath. Further processing involves a prewash, followed by a hypoclearing, a wash in filtered water, and a final methanol bath before drying. U.S. Pat. No. 4,265,534 describes a process of this type in detail.
The holographic approach offers a number of advantages. First, this method produces an encoder in which high-spatial-frequency errors are absent. Once the setup is in place, this method allows for the rapid replication of encoders, one after another, with relative ease. By use of this method, it is relatively easy to produce an encoder with a pitch on the order of a wavelength. When using this method, a xe2x80x9cfringe lockerxe2x80x9d can be used to stabilize the interference fringes for long exposures. The fringe locker is well known in the art, being a fringe stabilization system that views a part of the fringe pattern (above or below the hologram plate) and which electronically controls the relative phases of the two point sources so that the fringes remain locked in positionxe2x80x94thus compensating for air turbulence, vibrations, etc. that might otherwise move the fringe pattern.
A major disadvantage of this standard holographic approach, however, is the requirement for collimating optics whose diameters are on the order of the length of the desired encoder, and which require tight control of spherical aberration in order to maintain a uniform pitch over the entire length of the encoder. In the case of encoders in the order of one meter, the optics can be extremely expensive to produce.
The current invention provides a means for producing encoders of a uniform pitch, providing the advantages of the holographic method, without the need for the expensive collimating optics.